Cloth-like tissue sheets having camouflaged texture

ABSTRACT

Embossing regularly textured sheets with an appropriate regular, discrete embossing pattern to improve softness can result in a combined texture that creates an interference pattern that camouflages the original texture pattern and the embossing pattern. The resulting pattern is more appealing to the eye and is more random in appearance than the initial textured sheet or the embossing pattern individually. This result is particularly advantageous for paper towels.

BACKGROUND OF THE INVENTION

[0001] Two key attributes of a premium paper towel are softness and apleasing cloth-like visual texture. Softness can be enhanced byembossing the towel basesheet with a regular pattern of relativelysmall, discrete embossing elements, such as a pattern of dots. However,while the softness improvement is desirable, consumers tend to associateproducts having such regular embossing patterns with products of lowerquality. It would be desirable to be able to soften a paper towel, forexample, with an embossing pattern that is less objectionable to theconsumer and promotes a cloth-like appearance.

SUMMARY OF THE INVENTION

[0002] It has now been discovered that the negative visual impactassociated with embossing patterns having a regular pattern of discreteelements can be minimized by designing the embossing pattern tooptically interact with a pre-existing regular, distinct texture patternin the sheet to create an “interference pattern” that opticallycamouflages both the pre-existing texture pattern and the embossingpattern. The pre-existing texture pattern can be an embossing pattern orit can be a fabric imprinting pattern. This discovery has been found tobe particularly advantageous for embossing airlaid or throughdriedtissue sheets that have a regular, distinct surface texture imparted byone or more of the fabrics used to support the sheet during manufacture.

[0003] Hence, in one aspect the invention resides in a method ofembossing a textured tissue sheet having a regular, distinct, overalltexture pattern, said method comprising embossing the textured sheet toprovide a regular, distinct, overall embossing pattern that is differentthan the texture pattern and results in an optical interference pattern.

[0004] In another aspect, the invention resides in a tissue sheet havingat least two distinct, regular, overall texture patterns and an opticalinterference pattern.

[0005] In another aspect, the invention resides in a tissue sheet havinga surface texture characterized by 24 or fewer, more specifically 12 orfewer, still more specifically 6 or fewer, primary polar spatialfrequencies greater than 0.2 mm⁻¹ where the primary polar spatialfrequencies have Fourier magnitudes greater than 5 times the averageFourier magnitude for the tissue surface and are limited in number tothose with Fourier magnitudes of 20 percent or more of the specialfrequency with the largest Fourier magnitude, such that no two of theprimary Fourier magnitudes have absolute frequency differences less than0.1 mm⁻¹.

[0006] As used herein, the term “tissue sheet” is meant to include softand/or bulky paper sheets useful as facial tissue, bath tissue, papertowels or table napkins.

[0007] As used herein, an “optical interference pattern” is a patternthat is at least faintly discernable to the naked eye and results fromthe combination of two or more distinct, regular, overall texturepatterns that are at least slightly different in their pattern or intheir angular orientation. The optical interference pattern is theresult of adjacent regions on the surface of the sheet having differingdensities of visible pattern elements. For example, when two patternelements completely overlap each other, they appear as one element (lowdensity). When the same two elements fall side-by-side, they appear astwo closely-spaced elements (high density). These regions of differingtexture element densities gives rise to a new visible pattern thatcamouflages the appearance of the two individual patterns that createdit.

[0008] As used herein, the term “distinct” means that the patternconsists essentially of spaced-apart individual elements, such as dots,ovals, diamonds, squares and the like. The shape of the individualelements can be regular or irregular. The term “distinct” is intended todistinguish from patterns consisting of intersecting, relatively longcurvilinear lines.

[0009] As used herein, the term “regular” means that the pattern ofelements is repeating and is not random in at least one direction.

[0010] As used herein, the term “overall” means that the pattern ofelements substantially covers the sheet. Such patterns are sometimesreferred to as “background” patterns and are distinguished fromdecorative patterns consisting of relatively large spaced-apart iconssuch as flowers, butterflies, etc. Overall patterns will have from 4 toabout 50 elements per square centimeter, more specifically from about 10to about 30 elements per square centimeter, and still more specificallyfrom about 15 to about 20 elements per square centimeter. Also, in orderto be most effective for purposes of softening the sheet, overallembossing patterns will have a surface area coverage of from about 20 toabout 60 percent, more specifically from about 30 to 50 percent, andstill more specifically from about 35 to about 45 percent.

[0011] As used herein, the term “texture pattern” means a pattern ofelements having some three-dimensionality or a z-directional componentthat is noticeable to a user of the product. Texture can be imparted tothe sheet by embossing or during formation of the sheet by contact withvarious fabrics. The depth or z-directional component of the elements ofthe interfering texture patterns need to be the same or at leastsomewhat similar in magnitude, otherwise the optical interferencepattern will not be noticeable to a user of the product. Numerically,any difference in depth between the elements of the interfering patternsshould be about 80 percent or less, more specifically about 60 percentor less, still more specifically about 40 percent or less, still morespecifically about 20 percent or less, and still more specifically about10 percent or less.

[0012] The optical interference patterns can be formed by thecombination of two or more embossing element patterns or one or moreembossing element patterns in combination with a texture patternimparted to the tissue sheet when the sheet is made. In the latter case,it is common for tissue sheets, such as airlaid or throughdried sheets,to have a noticeable overall regular texture pattern of elements that isimparted to the sheet as a result of contact with a fabric duringmanufacture. The fabric can be a forming fabric, a transfer fabric, athroughdrying fabric or other fabric. These fabrics, if woven, have aregular knuckle pattern that imprints the sheet with texture elementsthat correspond to the knuckle pattern. In such cases, the subsequentembossing pattern can be designed to interact with the existing texturedsheet pattern. Methods of imparting initial texture element patterns tothe sheet while the sheet is being made are well known to those skilledin the tissue making art. Examples include, without limitation, methodsdisclosed in U.S. Pat. No. 6,017,417 entitled “Method of Making SoftTissue Products” issued Jan. 25, 2000 to Wendt et al. and U.S. Pat. No.5,935,381 entitled “Differential Density Cellulose Structure and ProcessFor Making Same” issued Aug. 10, 1999 to Trokhan et al., both of whichare herein incorporated by reference.

[0013] In order to generate an optical interference pattern, the two ormore embossing or texture element patterns must be different in some waywith regard to their application to the tissue sheet. This differencecan be in terms of the element spacing, the spacing of rows of elements,the angle of the rows of elements with respect to the machine directionof the tissue sheet, or the skewing of the pattern relative to thecross-machine direction of the sheet. Any one or more of these patterndifferences can give rise to an optical interference pattern.

[0014] In the simplest form, optical interference patterns can appear asa series of parallel stripes. In such cases, the thickness of thestripes can be from about 0.5 to about 3 centimeters, more specificallyfrom about 1 to about 2.5 centimeters, and still more specifically fromabout 1 to about 2 centimeters. If the optical interference pattern isthe result of the interaction of more than two distinct, regular overallpatterns, the optical interference pattern can manifest itself in theform of a regular pattern of odd shapes. It is possible to visuallymeasure the size and spacing of these patterns. However, because ofrandom effects also present on the tissue surface, a more precise methodof quantifying the presence of the interference patterns is by measuringthe surface topography of a large area of the tissue surface andtransforming that spatial distribution of the surface into a frequencydomain. This can be done in several different ways, but a method thatuses a mathematical transformation of the measured surface topography,specifically a Fourier transform, is particularly useful.

[0015] To carry out this method, a measurement of the tissue surface ismade on a 25 millimeter square section of tissue, although a larger sizeis also acceptable. The measurement records the height of the tissue ata regular orthogonal array of points that are equidistance from eachother, preferably less than 0.1 millimeter apart. The data is recordedas a two dimensional array consisting of the height of the tissue, z,measured in millimeters at each of the spatial (x,y) coordinates.

[0016] In order to provide the benefit of camouflage in the eyes of theproduct user, there must be multiple optical interference patternspresent on a particular tissue sheet product. The minimum number ofoptical interference patterns present will depend upon the size of thetissue sheet, the size and shape of the optical interference pattern andthe frequency of the optical interference pattern. For example, bathtissue sheets are typically only about 10 centimeters square. On theother hand, paper towel sheets are about 30 centimeters square. To beeffective, the number of optical interference patterns present in asingle sheet of bath tissue will be less than the number present in asingle sheet of paper toweling. In general, if the optical interferencepattern is a series of stripes, the number of stripes can be from about0.2 to about 1 per lineal centimeter, more specifically from about 0.3to about 0.9 per lineal centimeter, more specifically from about 0.4 toabout 0.8 per lineal centimeter, and still more specifically from about0.5 to about 0.7 per lineal centimeter, taken in a directionperpendicular to the direction of the stripes. Stated differently in amanner applicable to optical interference patterns of any shape, thepercent area of the tissue sheet occupied by an optical interferencepattern can be about 30 percent or greater, more specifically about 40percent or greater, still more specifically from about 30 to about 70percent, still more specifically from about 40 to about 60 percent, andstill more specifically from about 45 to about 55 percent.

[0017] As mentioned above, measurement of the area of an opticalinterference pattern can be made by visually approximating theboundaries of the optical interference pattern and simply calculatingthe percent area coverage. Alternatively, identification and measurementof the optical interference pattern can also be determined by the use ofsurface mapping and Fourier transform analysis. The analysis method isoutlined below:

[0018] 1. Measurement of surface topography over a 256×256 arraycovering at least 25×25 millimeters of tissue surface;

[0019] 2. Electronic conversion of X,Y,Z data scaled in millimeters to acomputer algorithm;

[0020] 3. Subtract the average value of the Z data from each Z element;

[0021] 4. Nyquist shift the Z data array;

[0022] 5. 2-D Fourier transform of the Z-data, converted to Fouriermagnitudes;

[0023] 6. Analyze the Fourier magnitudes and associated spatialfrequencies to find spatial frequency combinations that can lead to theformation of optical interference patterns.

[0024] The surface topography can be measured with a stylus profilometersuch as can be obtained using a Form Talysurf Laser InterferometricStylus Profilometer (Taylor Hobson Ltd., 2, New Star Road, Leicester,England LE4 9JQ). The stylus used is Part #112/1836, diamond tip ofnominal 2-micrometer radius. The stylus tip is drawn across the samplesurface at a speed of 0.5 millimeters/sec. The vertical (Z) range is6-millimeters, with vertical resolution of 10.0 nanometers over thisrange. Prior to data collection, the stylus is calibrated against ahighly polished tungsten carbide steel ball standard of known radius(22.0008 mm) and finish (Part # 112/1844 [Taylor Hobson Ltd.]. Duringmeasurement, the vertical position of the tip is detected by ahelium/neon laser interferometer pick-up, Part # 112/2033. Data iscollected and processed using Form Talysurf Ultra Series 2 software orequivalent.

[0025] To measure the topography parameters for a particular tissuesample, a portion of the tissue is removed with a single-edge razor orscissors (to avoid stretching the tissue) from a position near thecenter of the sheet (to avoid edge curl or other damage). The tissue isattached to the surface of a 2″×3″ glass slide using double-side tapeand lightly pressed into uniform contact with the tape using anotherslide. The slide is placed on the electrically operated, programmableY-axis stage of the profilometer. For purposes of measuring the surface,the profilometer is programmed to collect a “3D” topographic map,produced by automatically data logging 256 sequential profile traces inthe stylus traverse direction (X-axis), each 25 millimeters in length.The Y-axis stage is programmed to move in 98 micrometer increments aftereach traverse is completed and before the next traverse occurs,providing a total Y-axis measurement dimension of 25 millimeters and atotal mapped area measuring 25×25 millimeters. With this arrangement,data points each spaced 98 micrometers apart in both axes are collected,giving the maximum total 65,536 data points per map available with thissystem. The resultant “3D” topological map, being configured as a “.SUR”computer file consisting of X-, Y- and Z-axis spatial data (elevationmap), is then transformed into the frequency domain mathematically witha Fourier transform algorithm as described below. Other methods thatprovide a similar representation of the tissue surface, such as CADEYES(discussed in U.S. Pat. No. 5,779,965, which is herein incorporated byreference) may also be used.

[0026] The analysis of surface texture using Fourier analysis isdiscussed, for example, in the text The Image Processing Handbook, ThirdEdition, J. C. Russ, ISBN 0-8493-2532-3, and Development of Methods forthe Characterization of Roughness in Three Dimensions, K. J. Stout, ed.,ISBN 1 8571 8023 2, and Digital Image Processing, R. C. Gonzalez and P.Wintz, ISBN 0-201-11026-1, all of which are hereby incorporated byreference. Numerous software programs can be used to calculate theFourier transform and other data manipulations. National Instrumentsoffers one such software package (LabVIEW™) that is easy to use(National Instruments Corporation, 6504 Bridge Point Parkway, Austin,Tex. 78730-5039 (512) 794-0100.) The programming is graphical, and isshown listed in FIG. 10. This example program assumes the input heightdata from the surface topography measurement is stored in a 256×256array in a spreadsheet, and that the heights have all been normalized bysubtracting the mean of the data set from each element. Othernormalizations can also be made if necessary, such as removing anoverall tilt from one side of the tissue to the other due to improperleveling during measurement. Once the height data is stored, it isanalyzed as shown in FIG. 10.

[0027] Referring to FIG. 10, the first module reads the data in from aspreadsheet file, which the user must set-up from the surface topographyscan. Only the height data is read, as the x and y data are assumed tobe in numerical order of unscaled numbers 0-255. The second moduleNyquist shifts the data so that the resulting Fourier transform iscentered on zero frequency. Mathematically this is represented by

[0028] z(x,y)*exp(j2π(u₀x+v₀y)/N) for all pairs of x and y. u₀ and v₀have values of N/2, or 128 as defined here. The third module is the 2DFourier transform which follows the form:${F\left( {u,v} \right)} = {\frac{1}{N}{\sum\limits_{x = 0}^{N - 1}{\sum\limits_{y = 0}^{N - 1}{{z\left( {x,y} \right)}\quad {\exp \left( {j\quad 2\quad {{\pi \left( {{u\quad x} + {v\quad y}} \right)}/N}} \right)}}}}}$

[0029] where z(x,y) is the height data in the two orthogonal directions,x and y. N is the number of measurements in each direction, 256. F(u,v)is the Fourier transform of the height data. The independent variables uand v are no longer distances, but spatial frequencies in the x and ydirections, respectively. Because F is a complex number, the thirdmodule is a conversion from real and imaginary components to a polarcoordinate representation, such that for F(u,v)=Fr(u,v)+j*Fj(u,v) wherej is the square root of −1, Fr and Fj are the real and imaginarycomponents of F, and we calculate the Fourier magnitude, Fm, as thesquare root of (Fr*Fr+Fi*Fi) for each element of the 2-dimensionalarray, F. The final module writes the array to a spreadsheet file,although it could also be plotted, listed, or transferred to any othermedia.

[0030] Once the Fourier transform data is in a spreadsheet, it ishelpful to transform the 2-dimensional array into a 1-dimensional arrayand list the u and v coordinates next to the values of Fm. The u and vvalues are obtained by dividing the corresponding i and j indices thatrange from 0 to 255 by the size of the tissue sample, which in thisexample is 25 millimeters. The spreadsheet will then contain threecolumns of numbers, each column with 65536 numbers in it (256*256). Onecolumn will be the frequency in the x-direction (u values, the x indices0-255 divided by the sample size of 25 mm), one column will be thefrequency in the y-direction (v values, the y indices 0-255 divided bythe sample size of 25 mm) and the third column will be the Fouriermagnitude, Fm, of the 2-D Fourier transform of the surface topography.

[0031] The average Fourier magnitude of the surface is defined bycalculating the average value of all 65536 values of Fm. The u and vvalues that are associated with the 24 largest values of the 65536Fourier magnitudes are defined as the primary spatial frequencies. Theseare determined by sorting the entire data set in descending order. Oncethe average Fourier magnitude has been calculated and the entire dataset sorted, the lowest 65512 values of Fm and their associated values ofu and v can be deleted. For each of the 24 remaining Fourier magnitudesand the associated spatial frequencies u and v, only tissues where the24 primary spatial frequencies are greater than a predetermined minimumare considered. Because the two frequencies (u,v) for each primaryFourier magnitude can be different for the two directions, the spatialfrequencies are combined into a fourth variable, the polar spatialfrequency defined as the square-root of the sum of the squares of eachu,v frequency pair. Preferably this minimum polar frequency is 0.2 mm⁻¹.For many patterns, there are not 24 frequencies with large Fouriermagnitudes. In these cases, one should not use all of the largest 24Fourier magnitudes, but define a smaller subset of primary Fouriermagnitudes that contains the largest Fourier magnitude and all thoseFourier magnitudes smaller than the maximum that are larger than apredetermined percentage of the maximum, but always limited to a maximumnumber of 24 total. Specifically, this predetermined percentage can be20 percent or more, more specifically 30 percent or more, and still morespecifically 40 percent or more.

[0032] The smallest Fourier magnitudes also need to be significantlyhigher than the average level of all the Fourier magnitudes as definedabove. All of the primary Fourier magnitudes should have a value of 5 ormore times the average Fourier magnitude, more specifically 10 or moretimes the average Fourier magnitude, and even more specifically 20 ormore times the average Fourier magnitude.

[0033] For a regular pattern with primary polar frequencies of 0.2 mm⁻¹or greater, the absolute difference between any two pairs of spatialfrequencies that correspond to the primary Fourier magnitudes will be0.2 mm⁻¹ or greater. For the tissue disclosed here, there will be pairsof frequencies that are closer together than 0.2 mm⁻¹, which will resultin interference patterns to appear on the tissue sheet. The frequencydifference is calculated by comparing all possible combinations offrequencies, the absolute frequency difference being defined asfd=square root ((u_(i)−u_(j))²+(v_(i)−v_(j))²) where subscripts i and jrefer to any two different frequencies. For purposes of this invention,it is advantageous for this absolute frequency difference to be 0.1 mm⁻¹or less, more specifically 0.075 mm⁻¹ or less, and still morespecifically 0.05 mm⁻¹ or less, for at least one pair of the primaryFourier magnitudes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0034]FIG. 1 is a digital image of a dot-like pattern which replicates aknuckle imprinting pattern imparted to an airlaid tissue web duringmanufacture as described in the Example below.

[0035]FIG. 2 is a digital image of a dot-like pattern which replicatesan embossing pattern suitable for softening a tissue sheet.

[0036]FIG. 3 is a digital image of the combined pattern resulting fromoverlaying the dot-like pattern of FIG. 2 on top of the dot-like patternof FIG. 1, illustrating the camouflaging of the individual patterns andthe appearance of interference stripe patterns.

[0037]FIG. 4 is a photograph of an unembossed airlaid paper towel sheetillustrating the fabric imprinting pattern similar to the pattern ofFIG. 1.

[0038]FIG. 5 is a photograph of a smooth paper towel sheet embossed withan embossing pattern similar to the pattern of FIG. 2.

[0039]FIG. 6 is a photograph of a product of this invention, in whichthe airlaid paper towel sheet of FIG. 4 was embossed with the sameembossing pattern illustrated in FIG. 5, illustrating the opticalinterference pattern.

[0040]FIG. 7 is a schematic illustration of an airlaying formingapparatus suitable for making paper towels in accordance with thisinvention.

[0041]FIG. 8 is a schematic representation of an airlaying processsuitable for making paper towels in accordance with this invention.

[0042]FIG. 9 is 2-dimensional representation of the Fourier transformpeaks of an embossed tissue pattern in accordance with this invention.

[0043]FIG. 10 is a program listing of the National Instruments LabVIEWsoftware used to calculate the Fourier magnitudes and their associatedspatial frequencies in accordance with this invention.

DETAIL DESCRIPTION OF THE DRAWINGS

[0044] As used herein, the repeated use of any particular referencecharacter in different Figures is intended to represent the same oranalogous feature or element.

[0045] Referring now to FIG. 1, the invention will be described infurther detail. Shown is a digital image of a regular, distinct overallpattern of dots 2 (which can represent protrusions or depressions in atissue sheet) arranged in parallel rows running parallel to thecross-machine direction (CD) of the sheet. The dots in each alternatingrow are offset in the cross-machine direction by 25 percent of thespacing between the dots in the same row, resulting in an angular tiltto the pattern of about 25 degrees relative to the machine direction(MD) of the sheet. Also, as shown, the pattern as a whole isadditionally slightly skewed about 1 degree relative to thecross-machine direction of the sheet. The extent to which the pattern isskewed is illustrated by viewing the last continuous full row of dots inthe lowermost portion of FIG. 1.

[0046]FIG. 2 is a digital image of a regular, distinct overall patternof dots 3 which is different than the pattern of FIG. 1. In thispattern, the dots are arranged in rows parallel to the cross-machinedirection of the sheet (the pattern is square to the cross-machinedirection and is not skewed). The dots in adjacent rows are offset inthe cross-machine direction a distance of 50 percent of the spacingbetween dots in the same row, providing a staggered effect from row torow.

[0047]FIG. 3 is a digital image of a pattern which results fromoverlaying the pattern of FIG. 2 on top of the pattern of FIG. 1 or viceversa. Because the two individual patterns are different, a series ofoptical interference stripe patterns 5 is created. In this Figure, thereare six full-width optical interference stripe patterns illustrated. Allof the stripes are parallel to each other and are angled slightlyrelative to the machine direction of the sheet. The width “W” of eachoptical interference stripe pattern is about 1.2 centimeters andrepresents a region where the degree of overlap between elements 2 and 3is minimized, thereby appearing darker than the surrounding area wherethe some of the elements 2 and 3 overlap and appear as one. In thisexample, the spacing of the optical interference stripe patterns isabout 2 centimeters, center-to-center. The area coverage of the opticalinterference stripe patterns is about 50 percent.

[0048]FIG. 4 is a photograph of an airlaid paper towel sheet in which adistinct, regular, overall texture pattern of dots, corresponding to theknuckle pattern of a fabric used during manufacture, is imprinted intothe sheet. The geometry of this pattern is the substantially the same asthe pattern discussed above relative to FIG. 1. In the photograph, thedots represent depressions in the surface of the sheet.

[0049]FIG. 5 is a photograph of a smooth tissue sheet which has beenembossed with an embossing pattern as illustrated in FIG. 2 to produce aregular, distinct overall texture pattern. As shown in the photograph,the dots in the pattern represent depressions in the surface of thesheet.

[0050]FIG. 6 is a photograph of an airlaid paper towel sheet thatcontains the fabric imprinting dot pattern of FIG. 4 and which has beenembossed with the embossing pattern of FIG. 5. The resulting opticalinterference stripes 5 are indicated. They are less distinct than thoseillustrated in FIG. 3 because of the optical “noise” or clutterassociated with the fibers and texture of an actual sheet. Nevertheless,the optical interference stripe pattern is discernable.

[0051]FIG. 7 schematically illustrates an airlaying forming stationuseful for airlaying a web of fibers for making an airlaid sheet inaccordance with the Example below. As previously mentioned, there aredifferent ways of imparting texture patterns to the tissue sheet forpurposes of this invention. Fabric texture patterns associated withairlaying is one such method. Shown in FIG. 7 is an airlaying formingstation 30 which produces an airlaid web 32 on a forming fabric orscreen 34. The forming fabric 34 can be in the form of an endless beltmounted on support rollers 36 and 38. A suitable driving device, such asan electric motor 40 rotates at least one of the support rollers 38 in adirection indicated by the arrows at a selected speed. As a result, theforming fabric 34 moves in a machine direction indicated by the arrow42.

[0052] The forming fabric 34 can be provided in other forms as desired.For example, the forming fabric can be in the form of a circular drumwhich can be rotated using a motor as disclosed in U.S. Pat. No.4,666,647, U.S. Pat. No. 4,761,258, or U.S. Pat. No. 6,202,259, whichare incorporated herein by reference. The forming fabric 34 can be madeof various materials, such as plastic or metal.

[0053] As shown, the airlaying forming station 30 includes a formingchamber 44 having end walls and side walls. Within the forming chamber44 are a pair of material distributors 46 and 48 which distribute fibersand/or other particles inside the forming chamber 44 across the width ofthe chamber. The material distributors 46 and 48 can be, for instance,rotating cylindrical distributing screens.

[0054] In the embodiment shown in FIG. 7, a single forming chamber 44 isillustrated in association with the forming fabric 34. It should beunderstood, however, that more than one forming chamber can be includedin the system. By including multiple forming chambers, layered webs canbe formed in which each layer is made from the same or differentmaterials.

[0055] Airlaying forming stations as shown in FIG. 7 are availablecommercially through Dan-Webforming Int. LTD. of Aarhus, Denmark. Othersuitable airlaying forming systems are also available from M & JFibretech of Horsens, Denmark. As described above, however, any suitableairlaying forming system can be used in accordance with the presentinvention.

[0056] As shown in FIG. 7, below the airlaying forming station 30 is avacuum source 50, such as a conventional blower, for creating a selectedpressure differential through the forming chamber 44 to draw the fibrousmaterial against the forming fabric 34. If desired, a blower can also beincorporated into the forming chamber 44 for assisting in blowing thefibers down on to the forming fabric 34.

[0057] In one embodiment, the vacuum source 50 is a blower connected toa vacuum box 52 which is located below the forming chamber 44 and theforming fabric 34. The vacuum source 50 creates an airflow indicated bythe arrows positioned within the forming chamber 44. Various seals canbe used to increase the positive air pressure between the chamber andthe forming fabric surface.

[0058] During operation, typically a fiber stock is fed to one or moredefibrators (not shown) and fed to the material distributors 46 and 48.The material distributors distribute the fibers evenly throughout theforming chamber 44 as shown. Positive airflow created by the vacuumsource 50 and possibly an additional blower force the fibers onto theforming fabric 34 thereby forming an airlaid non-woven web 32.

[0059] The material that is deposited onto the forming fabric 34 willdepend upon the particular application. The fiber material that can beused to form the airlaid web 32, for instance, can include naturalfibers alone or in combination with synthetic fibers. Examples ofnatural fibers include wood pulp fibers, cotton fibers, wool fibers,silk fibers and the like, as well as combinations thereof. Syntheticfibers can include rayon fibers, polyolefin fibers, polyester fibers andthe like, as well as combinations thereof. Polyolefin fibers includepolypropylene fibers and polyethylene fibers. Synthetic fibers can bepresent, for instance, in an amount up to about 50% by weight, such asup to about 30% by weight of the furnish. The fibers can have variouslengths, such as up to about 6 to about 8 millimeters or greater.

[0060] When wood pulp fibers are present in the airlaid web of thepresent invention, the pulp fibers may be in a rolled and fluffed form.As is known to those skilled in the art, fluffed fibers generally referto fibers that have been shredded.

[0061] The pulp fibers used to form airlaid webs in accordance with thepresent invention may be pretreated with a debonding agent prior toincorporation into the airlaid web. Suitable debonding agents that maybe used in the present invention include cationic debonding agents suchas fatty dialkyl quaternary amine salts, mono fatty alkyl tertiary aminesalts, primary amine salts, imidazoline quaternary salts, siliconequaternary salt and unsaturated fatty alkyl amine salts. Other suitabledebonding agents are disclosed in U.S. Pat. No. 5,529,665 to Kaun whichis incorporated herein by reference. In particular, Kaun discloses theuse of cationic silicone compositions as debonding agents.

[0062] In one embodiment, the debonding agent can be an organicquaternary ammonium chloride and particularly a silicone based aminesalt of a quaternary ammonium chloride. For example, the debonding agentcan be PROSOFT TQ1003 marketed by the Hercules Corporation. Thedebonding agent can be added to a fiber slurry in an amount of fromabout 1 kg per metric tonne to about 6 kg per metric tonne of fiberspresent within the slurry.

[0063] When forming the airlaid web 32 from different materials andfibers, the forming chamber 44 can include multiple inlets for feedingthe materials to the chamber. Once in the chamber, the materials can bemixed together if desired. Alternatively, the different materials can beseparated into different layers in forming the web.

[0064] Referring to FIG. 8, a schematic diagram of an entire web formingsystem useful for making tissues or towels in accordance with thepresent invention is shown. In this embodiment, the system includesthree separate airlaying forming chambers 44A and 44B and 44C. Asdescribed above, the use of multiple forming chambers can serve tofacilitate formation of the airlaid web at a desired basis weight.Further, using multiple forming chambers can allow the formation oflayered webs. As shown, forming stations 44A, 44B and 44C contribute tothe formation of the airlaid web 32.

[0065] Airlaid web 32, after exiting the forming chambers 44A, 44B and44C, is conveyed on a forming fabric 34 to a compaction device 54A.Compaction device 54A can be, for instance, a pair of opposing rollsthat define a nip through which the airlaid web and forming fabric arepassed. For example, in one embodiment, the compaction device cancomprise a steel roll positioned above a rubber-coated roll. Thecompaction device moderately compacts the airlaid web to generatesufficient strength for transfer of the airlaid web to a transfer fabricsuch as, for instance, via an open gap arrangement. In general, thecompaction device increases the density of the web over the entiresurface area of the web as opposed to only creating localized highdensity areas.

[0066] After exiting the compaction device 54A, the airlaid web 32 istransferred to a transfer fabric 52. A suitable transfer fabric isElectroTech 56 manufactured by Albany International. Once placed uponthe transfer fabric, the airlaid web can be fed through a secondcompaction device 54B and further compacted against the transfer fabricto generate a texture pattern in the sheet. As previously described, theknuckle pattern of the transfer fabric can impart a texture pattern tothe web or sheet that can create an interference pattern when the sheetis subsequently embossed. The compaction device 54B can also be used toimprove the appearance of the web, to adjust the caliper of the web,and/or to increase the tensile strength of the web.

[0067] Next, the airlaid web 32 is transferred to a spray fabric 53A andfed to a spray chamber 56. Within the spray chamber 56, a bondingmaterial is applied to one side of the airlaid web 32. The bondingmaterial can be deposited on the top side of the web using, forinstance, spray nozzles. Under fabric vacuum may also be used toregulate and control penetration of the bonding material into the web.The bonding material can be applied to the web in order to add drystrength, wet strength, stretchability, and tear resistance.

[0068] In general, any suitable bonding material can be applied to theairlaid web 32. Particular bonding materials that may be used in thepresent invention include latex compositions, such as acrylates, vinylacetates, vinyl chlorides and methacrylates. Some water-soluble bondingmaterials may also be used including polyacrylamides, polyvinyl alcoholsand cellulose derivatives such as carboxymethyl cellulose. In oneembodiment, the bonding materials used in the process of the presentinvention comprise an ethylene vinyl acetate copolymer. In particular,the ethylene vinyl acetate copolymer can be cross-linked with N-methylacrylamide groups using an acid catalyst. Suitable acid catalystsinclude ammonium chloride, citric acid and maleic acid. Particularexamples of bonding materials that may be used in the present inventioninclude AIRFLEX EN1165 available from Air Products Inc. or ELITE PEBINDER available from National Starch. It is believed that both of theabove bonding materials are ethylene vinyl acetate copolymers.

[0069] The bonding material can be applied so as to uniformly cover theentire surface area of one side of the web. For instance, the bondingmaterial can be applied to the first side of the web so as to cover atleast about 80% of the surface area of one side of the web, such as atleast about 90% of the surface area of one side of the web. In otherembodiments, the bonding material can cover greater than about 95% ofthe surface area of one side of the web.

[0070] Once the bonding material is applied to one side of the web, asshown in FIG. 8, the airlaid web 32 is transferred to drying fabric 55Aand fed to a drying apparatus 58. In the drying apparatus 58, the web issubjected to heat causing the bonding material to dry and/or cure. Whenusing an ethylene vinyl acetate copolymer bonding material, the dryingapparatus can be heated to a temperature of from about 120° C. to about170° C.

[0071] From the drying apparatus 58, the airlaid web is then transferredto a second spray fabric 53B and fed to a second spray chamber 60. Inthe spray chamber 60, a second bonding material is applied to theuntreated side of the airlaid web. In general, the first bondingmaterial and the second bonding material can be different bondingmaterials or the same bonding material. The second bonding material maybe applied to the nonwoven web as described above with respect to thefirst bonding material.

[0072] From the second spray chamber 60, the nonwoven web is thentransferred to a second drying fabric 55B and passed through a seconddrying apparatus 62 for drying and/or curing the second bondingmaterial.

[0073] From the second drying apparatus 62, the airlaid web 32 istransferred to a return fabric 59 and may optionally be fed to a furthercompaction device 64 prior to being wound on a reel 66. The compactiondevice 64 can be similar to the first compaction device and maycomprise, for instance, calender rolls. Alternatively, the compactiondevice 64 can be a pair of embossing rolls used for the purpose ofsoftening and further texturizing the sheet and camouflaging the twotexture patterns as described above.

[0074] In order to emboss or further emboss the web 32 in accordancewith this invention, the web can subsequently be fed to an embossingstation. The embossing rolls can be any rolls suitable for embossingsuch as are well known in the art. Particularly suitable embossing rollscan be steel/rubber or steel/steel. Embossing nip pressures can be,without limitation, from about 100 to about 400 pounds per lineal inch.After embossing, the web can be conventionally converted into the finalproduct, which can be a paper towel, an industrial wiper, bath tissue,facial tissue, table napkin and the like.

[0075]FIG. 9 shows spatial frequencies of 12 primary Fourier magnitudesfor the pattern shown in FIG. 3. The symbols on the graph are thelocations of the 12 largest Fourier magnitudes in the spatial frequencydomain that are at least 30 percent of the largest Fourier magnitude.The x and y axes correspond to the spatial frequencies in the x and ydirection, respectively, and are in units of inverse millimeters (mm⁻¹).Each of the 12 symbols has the x-direction and y-direction frequencydisplayed next to it for clarity. The smallest polar spatial frequencyof the 12 primary Fourier magnitudes are associated with the foursymbols closest to the (0, 0) axis point at the center of the graph.There are four points, labeled (0.20, 0.12), (0.20,−0.12), (−0.20,0.12), (−0.20, −0.12) that all have the same value of the polar spatialfrequency, equal to 0.23 mm⁻¹. The spatial frequencies correspond to theinverse of the spacing of one of the underlying patterns or a harmonicof them. The circle around the point (0.20, −0.12) and the adjacentpoint (0.28, −0.12) is an example of how the two different base patternsthat formed the pattern in FIG. 3 result in frequency differencessmaller than the lowest primary polar frequency. The difference betweenthese two frequencies is 0.08 mm⁻¹, compared to 0.23 for the smallestpolar spatial frequency as shown above. The smaller frequencycorresponds to a larger period of repetition, in this case about 1.2 cm,which is larger than either of the base patterns. This will result in anoptical interference pattern in the tissue sheet of about this samescale.

[0076]FIG. 10 is a program listing of the National Instruments LabVIEWsoftware used to calculate the Fourier magnitudes and their associatedspatial frequencies in accordance with this invention. The first modulereads the data in from a spreadsheet file, which the user must set-upfrom the surface topography scan. Only the height data is read, as the xand y data are assumed to be in numerical order of unscaled numbers0-255. The second module Nyquist shifts the data so that the resultingFourier transform is centered on zero frequency. The third module is the2D Fourier transform, and the fourth module converts the complex Fouriercoefficients to a polar coordinate representation of real numbers. Thefinal module writes the array to a spreadsheet file, although it couldalso be plotted, listed, or transferred to any other media.

EXAMPLE

[0077] An airlaid paper towel basesheet was made in accordance with themethod described in FIG. 8. More specifically, Biobright TR kraft pulpfiber from UPM-Kymmene was fed to the three forming chambers. The fiberswere deposited onto the forming fabric traveling at a speed of about 800feet per minute. The weight percent ratio of fibers being deposited fromthe first, second and third forming chambers was 40/30/30. The basisweight was 55 grams per square meter. The newly-formed web wastransferred to a transfer fabric (Albany Electrotech 56) and, whilesupported by the transfer fabric, compacted in steel/rubber compactionnip. This compaction step imparted a knuckle pattern to the web asillustrated in FIG. 1. The latex binder (National Starch Elite PE) wasthereafter sprayed onto both sides of the compacted web at a totaladd-on level of about 12 percent and cured to above 95 percent at atemperature of about 170° C. The cured web was wound onto the reelwithout further compaction.

[0078] The resulting basesheet is shown in FIG. 4. The topographicalpattern imparted to the basesheet by the transfer fabric knuckle patternwas a regular pattern of depressions (dots) as digitally represented byFIG. 1, each dot being about 1 millimeter in diameter. The dots arearranged in a series of parallel rows substantially parallel to thecross-machine direction as described in connection with FIG. 1. Thespacing between dots within each row, center-to-center, is about 3.7millimeters. The spacing between rows, center-to-center, is about 2millimeters. Each row is offset from the adjacent row by 25 percent ofthe spacing between the dots, resulting in an angular tilt to thepattern of about 25 degrees.

[0079] Thereafter, the airlaid basesheet was embossed in accordance withthis invention. More particularly, the basesheet was passed through arubber/steel embossing nip at ambient temperature with a nip pressure ofabout 300 pounds per lineal inch. The rubber backing roll had a hardnessof 65 Shore A. The surface of the engraved steel roll had a regularpattern of protrusions as shown in FIG. 2. The protrusions had a lengthof 3 millimeters and a width of 1.5 millimeters. The protrusions werearranged in parallel rows diagonal to the machine direction of thebasesheet. The spacing between the rows, center-to-center, was 4.0millimeters and the spacing between elements, center-to-center, was 5.1millimeters. Each row is offset from the previous row by 50 percent ofthe spacing between the dots, resulting in an angular tilt to thepattern of about 32 degrees. The resulting basesheet is shown in FIG. 6.A digital representation is shown in FIG. 3.

[0080] In the interests of brevity and conciseness, any ranges of valuesset forth in this specification are to be construed as writtendescription support for claims reciting any sub-ranges having endpointswhich are whole number values within the specified range in question. Byway of a hypothetical illustrative example, a disclosure in thisspecification of a range of from about 1 to about 5 shall be consideredto support claims to any of the following sub-ranges: 1-4; 1-3; 1-2;2-5; 2-4; 2-3; 3-5; 3-4; and 4-5.

[0081] It will be appreciated that the foregoing description andexample, given for purposes of illustration, are not to be construed aslimiting the scope of the invention, which is defined by the followingclaims and all equivalents thereto.

We claim:
 1. A method of embossing a textured tissue sheet having aregular, distinct, overall texture pattern, said method comprisingembossing the textured sheet to provide a regular, distinct, overallembossing pattern that is different than the texture pattern and resultsin an optical interference pattern.
 2. The method of claim 1 wherein thetexture pattern is a fabric pattern imprinted into the sheet while thesheet is being made.
 3. The method of claim 1 wherein the texturepattern and the embossing pattern comprise rows of distinct elements,wherein the embossing pattern differs from the texture pattern withrespect to the spacing of the elements within the rows.
 4. The method ofclaim 1 wherein the texture pattern and the embossing pattern compriserows of distinct elements, wherein the embossing pattern differs fromthe texture pattern with respect to the spacing of the rows elements. 5.The method of claim 1 wherein the texture pattern and the embossingpattern comprise rows of distinct elements, wherein the embossingpattern differs from the texture pattern with respect to the orientationof the rows of elements relative to the machine direction of the sheet.6. The method of claim 1 wherein the sheet is airlaid.
 7. The method ofclaim 6 wherein the texture pattern is a fabric imprint pattern.
 8. Themethod of claim 1 wherein the sheet is wet laid and throughdried.
 9. Themethod of claim 8 wherein the texture pattern is a fabric imprintpattern.
 10. The method of claim 1 wherein the texture pattern is anembossing pattern.
 11. A method of embossing an airlaid tissue sheethaving a regular, distinct, overall fabric texture pattern imparted tothe sheet during manufacture, said method comprising embossing thetextured sheet to provide a regular, distinct, overall embossing patternthat is different than the fabric texture pattern and which results inan optical interference pattern.
 12. A method of embossing a tissuesheet comprising embossing the tissue sheet to produce a regular,distinct, overall first texture pattern and thereafter embossing thesheet to provide a regular, distinct, overall second texture patternthat is different than the first texture pattern and which results in anoptical interference pattern.
 13. A tissue sheet having at least twodistinct, regular, overall texture patterns and an optical interferencepattern.
 14. The tissue sheet of claim 13 wherein the interferencepattern comprises multiple parallel stripes.
 15. The tissue sheet ofclaim 13 wherein the thickness of the stripes is from about 0.5 to about3 centimeters.
 16. The tissue sheet of claim 13 wherein the thickness ofthe stripes is from about 1 to about 2 centimeters.
 17. The tissue sheetof claim 13 wherein the percent area of the tissue sheet occupied by theinterference pattern is about 30 percent or greater.
 18. The tissuesheet of claim 13 wherein the percent area of the tissue sheet occupiedby the interference pattern is about 40 percent or greater.
 19. Thetissue sheet of claim 13 wherein the percent area of the tissue sheetoccupied by the interference pattern is from about 30 to about 70percent.
 20. The tissue sheet of claim 13 wherein the percent area ofthe tissue sheet occupied by the interference pattern is from about 40to about 60 percent.
 21. The tissue sheet of claim 13 wherein thepercent area of the tissue sheet occupied by the interference pattern isfrom about 45 to about 55 percent.
 22. A tissue sheet having a surfacetexture characterized by 24 or fewer primary polar spatial frequenciesgreater than 0.2 mm⁻¹ where the primary polar spatial frequencies haveFourier magnitudes greater than 5 times the average Fourier magnitudefor the tissue surface and are limited in number to those with Fouriermagnitudes of 20 percent or more of the special frequency with thelargest Fourier magnitude, such that no two of the primary Fouriermagnitudes have absolute frequency differences less than 0.1 mm⁻¹. 23.The tissue sheet of claim 22 wherein the number of primary polar spatialfrequencies is 12 or fewer.
 24. The tissue sheet of claim 22 wherein theprimary polar spatial frequencies are limited in number to those withmagnitudes of 30 percent or more of the spatial frequency with thelargest magnitude.
 25. The tissue sheet of claim 22 wherein the primarypolar spatial frequencies are limited in number to those with magnitudesof 40 percent or more of the spatial frequency with the largestmagnitude.
 26. The tissue sheet of claim 22, 23, 24 or 25 wherein theprimary polar spatial frequencies all have Fourier magnitudes 10 or moretimes the average Fourier magnitude for the tissue surface.
 27. Thetissue sheet of claim 22, 23, 24 or 25 wherein the primary polar spatialfrequencies all have Fourier magnitudes 20 or more times the averageFourier magnitude for the tissue surface.
 28. The tissue sheet of claim22 wherein no two of the primary Fourier magnitudes have absolutefrequency differences of 0.075 mm⁻¹ or less.
 29. The tissue sheet ofclaim 22 wherein no two of the primary Fourier magnitudes have absolutefrequency differences of 0.05 mm⁻¹ or less.
 30. The tissue sheet ofclaim 22 wherein the sheet is an airlaid sheet.